Production of oxygen-free copper



Patented Aug. 16, 1949 I UNITED STATES PATENT 'OFFICE PRODUCTION OF OXYGEN-FREE COPPER Arthur L. Christensen, Perth Amboy, N. J., and

Horace F. Silliman, Waterbury, Conn., assignors to International smelting and Refining Company, a corporation of Montana No Drawing. Application July 11, 1945,

Serial No. 604,500

2 Claims. (oi. 75-76) This invention relates to the production of must be avoided and the residual excess of the oxygen-free copper and has for its object the prodegassing or deoxidizing element must be very vision of an improved method of producing subcarefully controlled. Although boron has less ef-' stantially oxygen-free copper of high electrical feet on reducing the conductivity of copper than conductivity. 7 most other elements, it has'nevertheless sufiicient The present invention is a variant of the invenefi'ect so that it must be maintained in the copper tion disclosed in the copending application of at less than 0.02%, if the A. S. T. M. specification Christensen, McKean' and Silliman Ser. No. for conductivity is to'be met. 560,898, filed October 28, 1944, now abandoned,' Smooth deposition of copper in an electrolytic wherein boron is used to degas copper rather than 10 refinery is obtained by including in the copper to deoxidize it. As pointed out in that applica-' electrolyte certain colloidal addition agents which, tion, it is extremely difiicult to melt copper withto some extent, become imbedded in the deposited out picking up some oxygen. This is true even cathode copper. Animal glue, a Wood pulp mill in an induction furnace, because of the exposure residue known as goulac, and some so-called water to the atmosphere of projecting hot ends of the soluble mineral oils are examples 'of suchaddicopper before it is melted down. Inthe process tion agents. "High quality copper cathodes show of the aforementioned application, oxygen in the an ignition loss of 0.015%of which only 0.0015% molten copper is removed by treatingthe copper is sulphur. The remainder is laregly organic with a reducing gas such as hydrogen which dismatter that breaks down in molten copper to solves in the copper and removes any dissolved various other compounds of carbon, hydrogen, oxygen, and the residual dissolved hydrogen is oxygen and nitrogen with residual hydrogen, hy-

then removed by treatment with boron. In acdrogen sulphide and probably other reducing cordance W e p t on, C pper is gaseous impurities remaining in the molten copmelted down in contact with carbon or graphite per. The heretofore generally accepted practice heated to a temperature above the melting temof producing sound oxygen-freecopper is'to exp of pp and the exposed Surface O e pose the molten cathode copper to air with the o t c pper is protected W th a cove of fin result that a certain oxygen pick-up in the copper carbon or graphite. The molten copper eflects a removal these reducing gaseous imis effectively deoxidized by the action thereon of p jtje The oxygen is th removed w t the hot carbon graphite- The resulting cined charcoal, carbon monoxide free of water oxidized moltencopper (preferably at a tempera- Vapor and y g n or by the use of some ture of about 2200 F.) is then treated with boron xidizing l nt su h as phosphorus, lithium, to remove any dlssolved reducmg gasesboron, etc. Any substantial residual excess of Elecirolyglcany g coppel: contains 1 3 M the deoxidizingelement lowers the electrical con- 1 3 i iggs ig gfi i gg a g it; ductivity of the copper, and hence it is diflicult to 0 002% with arsenic and antimony present in the econommany produce hlghconductmty lesser amounts. When making tough pitch wire fre-e copper by these pragtlce's' contrastgd wlth bars this copper has an electrical conductivity thls helietoior? generally zicqepted practlce the ranging from 100.5% to 101.5% I. A. c. s. It con- 40 present mventmnfirst demdlzes the to as cuprous oxide on the grain i but this ing gases resulting from the decomposition oi oxygen has placed some of the Impurities such as occluded addition agents, electrolyte and moisarsenic, antimony, iron or nickel on the grain boundaries as well. Tough pitch copper with this tum The reducmg are t rammed-by the boron treatment, which requires less boron conductivity can be treated with carbon to remove all the oxygen, cast and drawn into oxygen-free t 15 reqmred'when boron 15 ,used w copper wire which will have a conductivity rangmg e Moreover mYentlon provides mg from 100.0 to 101.0% The removal of oxygen more precise control over the residual boron conhas placed the impurities in solution in the copper tent of the copper with the result of 8. 0.5% drop in conductivity. Somemetals have'the Property of Picking up The A. S. T. M. specifications for high conductivcarbon, an? Where a pick-up has to be ity copper include the requirement of a avoided it is not possible to take advantage of conductivity. Generally the refiner setshis own F P induction. furnace heating e o e requirement at 100.5% allowing himself a 0.5% obtammg when using a graphite c t s factor of safety because of minute errors and been found vthat pp o picks p f O variations that can develop in the preparation but only in y Small 2111101111118 andlmder Special and testing of the wire. It is therefore evident d t QIIS. Co sequently, for all practical D from the foregoing figures that when degassing pos s copper is not contaminated by me in or deoxidizing copper any form of contamination 139 graphite or carbon. The present invention takes advantage or this circumstance, and' contemplates melting copper in a. carbon orgraphite crucible or the like heated to a temperature above the melting temperature of copper and protecting the exposed surface of the resulting molten. copper by a cover of finely divided or pulverized carbon or graphite. Since the carbon. or raphite crucible is hotter than the molten; coppen. any oxygen in the copper is effectively: removed; andi the copper is thereby deoxidized, by contact with the hot carbon or graphite. This. deQX-idation so efiective and complete that no special treat-- ment of the copper with reducing gases is necessary prior to the boron treatment. As soon as all the copper is molten the surface of the molten copper is, covered witnpowdered carbon or graphiteso-as to avoid contact; withthe air, and there is then no: possibility for reoxidation. of the cops per. At this stage, the molten copper is oxygenfroe, but it: contains dissolved: reducing gases. In accordance with. the present. invention, these re.- duciligf gases are then removed from the molten copper by treatment with a boron-containing a ent. preferably a, boron-copperalloy; in amount icsuflicient to introduce into the treated copper such an amount. of residual boronasto objectionabl-y reduce. its electrical conductivity;

The reducing gases in the deoxidized molten copper originate for the; most. part from impurities in the copper. Cathode copper is a particuiarly desirable form of copper for the production of oxygen-free high conductivity copper, and customarily contains traces oi organic matter and traces of occluded electrolyte and: moisture. In

the. course oi the: melt down. and deoxidation of the. copper by contact with hot; carbon or graphite, the impurities are. largely converted to, reducing gases such as. hydrogen, hydrogen sulphide and probably some hydrocarbons which may not crack at the prevailing; temperature. These re.- ducing gases are dissolved in the oxygen-tree molten copper and are effectively removed b the boron treatment. Since the copper contains. no oxygen, the boron is not a. deoXi-dizing agent, and only a relatively small amount is. required to remove the dissolved reducing gases. From 0.005 to 0.02% oi boron based on. the weight of the molten copper treated is sufiicient for the purpose, and ordinarily about 0.01 boron by weight. gives entirely satisfactory results. The. boron is pref erably introduced into the molten copper in the form of a master alloy of boron and copper con.- taining from 0.5 to boron and the balance copper with less than 1% impurities. Such a boron-copper alloy admirably adapted for the practice of the invention is made in accordance with Silliman Patent. No. 2,195,433,. and contains 3 to 5% boron, around 95% copper and less than 1% impurities (mostly iron, silicon and magnesium).

In practicing the invention, the. copper is. preferably melted in a graphite crucible of a coreless induction furnace. With a graphite crucible, the furnace operates under a very steady power load because the crucible absorbs all the eddy currents. The crucible becomes the hottest part of the furnace and the copper is melted entirely by conduction of heat from the crucible. Graphite is the preferred form of carbon because it is more dense and burns away less rapidly than other available forms of carbon. The graphite or other form of carbon should be of high purity such for example as high quality Acheson graphite. Although induction heating is preferred, the graphite or carbon crucible may be locally heated 1 graphite paste bonded joints.

in any: app priate manner. as for; e ample by electric resistance heating units.

The crucible or the like may be made of or lined with carbon or graphite in various Ways. In a small. 001161.855. induction furnace, the crucible may be lined with a plastic graphite mix containing a. carbonaceous binder, such, for example, as pitch orsugarsolution. In larger furnaces, prefio-rmed: circle block linings may be used, with It is our present preferred practice to turn out the crucible from high. quality Acheson graphite electrodes. Such electrodes are now available with diameters up to 24- inches. A suitable crucible wall thickness is 3 inches and it is thus possible to develop inside dimensions of 1.8. inches. in diameter by about 3.0" inches: high which is a shape that allows a satisfactory'furnacedesign. Such a crucible can produce about 2000:- pounds of oxygen-free. high conductivity copper per heat in a period or to. 9.0, minutes;

In addition to its action in deoxidizing the molten copper, the carbon orgraphite crucible is of special advantage with respect to. the. boron treatment, .of' the molten copper. Boron is an extremely energetic reducingagent, and reacts with the metal oxides. and silicates commonly present in many furnace refractories, thereby forming free metals (e. g. silicon and iron). which very deleteriously affect. the electricat conductivity of the copper. For example, small amounts. of silicon and" iron, say 0.005% of each, have a very noticeable detrimental effect on the conductivity or copper. Carbon and graphite are not only resistant to this action of boron, but are available in such pure state as not to contaminate the boron-treated copper.

The following. example illustrates an actual practice of the invention in a 1.25 kw. coreless induction furnace having a crucible turned out of a highestv quality Acheson graphite electrode. The boron treatment was carried out with a. boronecopper alloy containing 3% boron, 0.6% impurities consisting mainly of magnesium, silicon and iron, and the remainder copper. The steps in the process. were as follows-:-

(1) After casting out the previous charge oi boron-treated copper, the crucible was immedn atelyfilled with cut. copper cathodes ot a size which stacked in the crucible suflici'ently well so as tov require only about two. additional recharge ings in order to finally develop a full crucible of molten copper. Prior to the start of recharging and during this entire melting period the full load of the motor generator was on the furnace coil. In this. case the watt meter on the output side of the generator showed to kw. The load was practically constant throughout the ens tire period because the crucible was drawingall the power and as a result of this condition it was not necessary to make the power adjustments normally required when using a less conductive crucible. During this melt-down period pulverized Acheson graphite varying in size from 20 mesh to minus 200 mesh was added so as to cover the bath of copper. When the crucible was partly full of molten copper the charged cathodes would submerge and there was bubbling from traces of organic matter and traces of occluded electrolyte and moisture previously mentioned.

(2) In 50 minutes time the copper was entirely molten and as the last piece melted the power was immediately shut 01f. The crucible was still much hotter than the copper but in a few minutes time the temperature was uniform. More p111:

verized graphite was placed on any exposed surface of the bath. The temperature was then taken with an optical pyrometer sighting into a closed end target tube submerged in the bath. If the temperature was 2200 F. treatment with the master alloy was made. If less than 2200" F. some additional heating was done.

(3) During this period the copper molds had been stripped from the previous castings, cooled to about 200 F. with water, then placed in their normal vertical position on rails and spray-coated with a water dispersion of finely ground Dixon graphite and lampblack in a ratio of about 8-5-1-0.2 water-graphite-lampblack-tannic acid by weight. With a few minutes of standing with bottoms open on rails the molds were dry and they were then placed on the cast iron base plates in line to receive the treated copper. The base plates were not dressed. Cleaner, smoother surfaces were obtained by not dressing the base plates.

(4) The copper in the furnace crucible was treated with the required boron by wiring paper bags containing the boron-copper alloy (3% boron) to graphite rods, submerging under the powdered graphite cover and stirring until dissolved. This stirring required to seconds.

(5) In order to prevent air from sweeping into the crucible, while tipping to cast, a clay-graphite crucible cover was placed on the crucible. This cover was 4 to 6 inches away from the molten copper but it prevented a sweep of air into the crucible. confined and maintained a carbon monoxide atmosphere over the bath as a result of the burning powdered graphite. This cover minimized boron losses during casting.

(6) The crucible was tipped until just ready to pour and then the powdered graphite was pushed back from the lip sufficiently to get a clean pour started. Thereafter the graphite did not travel out of the crucible with the copper but remained back toward the bottom of the crucible and it was possible to empty the copper completely without a detrimental amount of graphite carrying out into the mold. This inert and slightly cohesive character of Acheson graphite powder makes it an ideal cover for molten copper. Charcoal and other carbonaceous materials generally require calcination prior to use in this way in order to avoid gassing the copper. Dry Acheson graphite requires no treatment prior to use. It is also maintained on the bath in a more easily controlled manner and does not require a bottom pour ladle or any other special means to prevent carrying over into the molds.

(7) When the one mold had been filled with copper it was necessary to wait until freezing had started and refill once or twice, depending on the size of the casting, so as to make up for the shrinkage which occurs when copper passes from the molten to the solid state. This casting and filling of the shrink hole required 3 to 5 minutes for each casting. The empty furnace was then tipped back to melting position, the clay-graphite cover was removed and recharging of cathodes was started with the power on the furnace as The copper produced in the foregoing example was oxygen-free, possessed high electrical conductivity, superior working properties, and the high density characteristic of high quality borontreated copper. The boron-copper alloy was used in three different amounts so as to treat the copper with 0.01%, 0.015% and 0.02% by weight of boron. The degree of control obtained by the induction furnace heating was so precise that the electrical conductivities of castings resulting from an 0.02% boron treatment were 0.65% lower than castings resulting from an 0.01% boron treatment. The average results of nine castings from each of the three boron treatments were as follows:

Per cent Boron Added 0.01 0.015 0.02

Average Per Cent Conductivities 100.67 100. 25 100.02 Average Bends after 30 min. H anneaL l3. 6 14. 2 15. 0 Average Densities 8.188 to 8. 9'1

The foregoing results indicate aprecision of control not obtainable with any of the heretofore generally accepted practices for producing oxygen-free high conductivity copper. Oxygen-free copper with a residual boron content of 0.001 to 0.01% (and usually about 0.005%) can be consistently and economically produced by the invention.

We claim:

1. The method of producing substantially OX- ygenfree copper which comprises melting copper in contact with carbon heated to a temperature above the melting temperature of copper and protecting the exposed surface of the resulting molten copper with a cover of finely divided carbon for a'suflicient period of time for the molten copper to become substantially completely deoxidized by contact with the hot carbon and to absorb reducing gases, and then removing the reducing gases from the resulting substantially completely deoxidized molten copper by treatment thereof with a boron-containing agent of the group consisting of boron and boron-copper.

2. The method of producing substantially oxygen-free copper which comprises melting copper in a carbon crucible heated to a temperature above the melting temperature of copper and protecting the exposed surface of the resulting molten copper with a cover of finely divided carbon and substantially completely deoxidizing the molten copper by the action thereon of the hot carbon of the crucible and with the absorption therein of reducing gases, and then removing the reducing gases from the resulting substantially completely deoxidized molten copper by treatment thereof with a boron-containing agent of the group consisting of boron and boron-copper in such amount as to introduce into the treated copper less than 0.02% of residual boron.

ARTHUR L. CHRISTENSEN. HORACE F. SILLIMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,023,604 Weintraub Apr. 16, 1912 1,578,044 Lapsley Mar. 23, 1926 1,955,726 Archer et al Apr. 24, 1934 2,003,889 Jennison et al June 4, 1935 2,195,433 Sill-i-man Apr. 2, 1940 FOREIGN PATENTS Number Country Date 423,697 Great Britain Feb. 6, 1935 OTHER REFERENCES Transactions, A. I. M. M. E., vol. 104, pages 152 to (1935). r

1001 Alloy Formulas, by Jarvis 1927, pages 25 and 26. 

